Tractor feed mechanisms commonly used in a printer apparatus comprise a continuous flexible pin belt wrapped around a pair of drive sprockets or pulleys rotatably supported by a frame assembly or body mountable on drive and guide bars of the paper feed assembly of the printer. In addition to supporting the pin belt and drive sprockets, the tractor body has surfaces for guiding paper so that perforations near the edges of the paper are in proper alignment to receive and be engaged by drive pins carried by the belt as it is rotated by the sprockets to feed the paper past a print station of the printer. The paper may be a continuous web and may have multiple layers in which case it is commonly called a multipart form and the tractor feed mechanism is referred to as a forms feed tractor.
It is common practice to provide such feed mechanisms with a door or lid for constraining the paper so that it remains in contact with the guide surface of the tractor body and does not become disengaged from the pins. The door is provided with a hinge connection to the body. This enables the door to be rotated around the hinge axis between a closed position where the paper is constrained to remain on the pins and an open position to allow paper to be placed on or removed from the tractor. A spring is used to hold the door in either the closed or open position. In a copending application Ser. No. 150,348 filed on 01/29/89, now U.S. Pat. No. 4,817,842, issued on Apr. 4, 1989; the force for holding the door in the closed position is provided by a flat leaf spring which is bent so as to be spring loaded against the curved surface of door position cam control means rotatable with the door. Other tractors well known in the art use coil springs connected to the door and tractor body to provide the door constraining force.
When perforated webs are fed at high speeds and accelerations, there is a tendency for the forms to slide up the side of the feed pins toward the door. This motion is caused by the vibrations induced in the paper web due to high acceleration forces. The sliding effect is aggravated by changes in thickness of the paper due to folds. The condition is worsened where multilayer forms are used. The sliding motion on the pins causes abrasion of the feed pins and causes the registration of the forms to vary with respect to the position of forms feed tractors. When the lift force is strong enough, it causes the door itself to move up and down in a fluttering motion. Conceivably, the force might reach a point where the door itself might fly open and the forms would disengage from the feed pins.
Various attempts have made to maintain perforated paper on the pins. One such attempt uses a pressure belt which is pressed onto the pins of the belt above the paper. Such arrangements may be seen in U.S. Pat. Nos. 3,209,972; 3,608,801; and 3,669,329. A like tractor is used in the IBM 4248 Printer. Such tractors while effective tend to be complex and costly devices.
Another attempt to solve the paper lifting problem is to provide a structure in which the paper gap between the door and the top of the surface of the pin belt is as close as possible to the top of the paper. In fact, the gap is made so small that the door may actually contact the surface of the paper. U.S. Pat. No. 4,611,737 shows such a tractor mechanism. This requires a door stop mechanism and in order to maintain such fine dimensional spacing very precise manufacture is involved which can be difficult to achieve and is costly. A further problem with such a structure is that relatively high forces are required to prevent the forms from disengaging from the pins when folds in multilayer forms pass through the door gap. A consequence of this is that unnecessarily high door to paper forces are required by that design which generates high system friction, increases the wear rate of the tractor door and body, and causes deformation of the perforations in the forms. Another problem produced with the design of the mentioned patent is that the pressure on the paper in the region where the paper is disengaging from the tractor feed pins impedes the stripping of the paper from the pins and paper jams result.